I would like to describe a change in viewpoint, which I believe will alter how we think about everything from the most abstract questions on the nature of truth to the most concrete questions in our daily lives. This change comes from the deepest and most difficult problems facing contemporary science: those having to do with the nature of time.

The problem of time confronts us at every key juncture in fundamental physics: What was the big bang and could something have come before it? What is the nature of quantum physics and how does it unify with relativity theory? Why are the laws of physics we observe the true laws, rather than other possible laws? Might the laws have evolved from different laws in the past?

After a lot of discussion and argument, it is becoming clear to me that these key questions in fundamental physics come down to a very simple choice, having to do with the answers to two simple questions: What is real? And what is true?

Many philosophies and religions offer answers to these questions, and most give the same answer: reality and truth transcend time. If something is real, it has a reality which continues forever, and if something is true, it is not just true now, it was always true, and will always be. The experience we have of the world existing within a flow of time is, according to some religions and many contemporary physicists and philosophers, an illusion. Behind that illusion is a timeless reality, in modern parlance, the block universe. Another manifestation of this ancient view is the currently popular idea that time is an emergent quality not present in the fundamental formulation of physics.

The new viewpoint is the direct opposite. It asserts that what is real is only what is real in the moment, which is one of a succession of moments. It is the same for truth: what is true is only what is true in the moment. There are no transcendent, timeless truths.

There is also no past. The past only lives as part of the present, to the extent that it gives us evidence of past events. And the future is not yet real, which means that it is open and full of possibilities, only a small set of which will be realized. Nor, on this view, is there any possibility of other universes. All that exists must be part of this universe, which we find ourselves in, at this moment.

This view changes everything, beginning with how we think of mathematics. On this view there can be no timeless, Platonic, realm of mathematical objects. The truths of mathematics, once discovered, are certainly objective. But mathematical systems have to be invented-or evoked- by us. Once brought into being, there are an infinite number of facts which are true about mathematical objects, which further investigation might discover. There are an infinite number of possible axiomatic systems that we might so evoke and explore-but the fact that different people will agree on what has been shown about them does not imply that they existed, before we evoked them.

I used to think that the goal of physics was the discovery of a timeless mathematical equation that was isomorphic to the history of the universe. But if there is no Platonic realm of timeless mathematical object, this is just a fantasy. Science is then only about what we can discover is true in the one real universe we find ourselves in.

More specifically, this view challenges how we think about cosmology. It opens up new ways to approach the deepest questions, such as why the laws we observe are true, and not others, and what determined the initial conditions of the universe. The philosopher Charles Sanders Pierce wrote in 1893 that the only way of accounting for which laws were true would be through a mechanics of evolution, and I believe this remains true today. But the evolution of laws requires time to be real. Furthermore, there is, I believe, evidence on technical grounds that the correct formulations of quantum gravity and cosmology will require the postulate that time is real and fundamental.

But the implications of this view will be far broader. For example, in neoclassical, economic theory, which is anchored in the study of equilibria of markets and games, time is largely abstracted away. The fundamental results on equilibria by Arrow and Debreu assume that there are fixed and specifiable lists of goods, and strategies, and that each consumer’s tastes and preferences are unchanging.

But can this be completely correct, if growth is driven by opportunities that suddenly appear from unpredictable discoveries of new products, new strategies, and new modes of organization? Getting economic theory right has implications for a wide range of policy decisions, and how time is treated is a key issue. An economics that assumes that we cannot predict key innovations must be very different from one that assumes all is knowable at any time.

The view that time is real and truth is situated within the moment further implies that there is no timeless arbiter of meaning, and no transcendent or absolute source of values or ethics. Meaning, values and ethics are all things that we humans project into the world. Without us, they don’t exist.

This means that we have tremendous responsibilities. Both mathematics and society are highly constrained, but within those constraints there are an infinitude of possibilities, only a few of which can be evoked and explored in the finite time we have. Because time is real and the future does not yet exist, the imaginative and social worlds in which we will live are to be brought into being by the choices we will make.